1. Field of the Invention
The present invention relates to laser-based imaging systems which are used for time-gated imaging, imaging through turbid media, optical sectioning, metrology, image amplification, frequency conversion of images, and confocal microscopy.
2. Description of the Related Art
Research relating to optical parametric image amplification has concerned the upconversion of weak infrared images, and the selective amplification of certain spatial frequencies without regard to time resolution. As reported by J. Watson et al. in xe2x80x9cImaging in diffuse media with ultrafast degenerate optical parametric amplification,xe2x80x9d Optics Letters, Vol. 20, p. 231 (1995), time-resolved, degenerate optical parametric image amplification has been used for transillumination imaging through turbid media by providing a sub-picosecond time gate to temporally discriminate against scattered photons. This method, however, does not provide optical sectioning of the object or surface contour information. These optical parametric amplification (OPA) imaging techniques have employed the OPA either at an image plane or in the Fourier plane of the optical system.
Ultrafast time-gated imaging has also been employed to observe fast processes, such as the propagation of light pulses through various media. Time-gating has been performed using techniques other than optical parametric amplification. These techniques include: LIF holography, as disclosed by J. A. Valdmanis et al. in xe2x80x9cThree-dimensional imaging with femtosecond optical pulses,xe2x80x9d Optical Society of America, Conference on Lasers and Electro-Optics, Vol. 7, paper CTUA1, (1991); picosecond Kerr shutters, as disclosed by M. A. Duguay et al. in xe2x80x9cUltrahigh speed photography of picosecond light pulses and echoes,xe2x80x9d Appl. Opt., Vol. 10, pp. 2162-2170 (1970) and by L. Wang, et al. in Science, Vol. 253, p. 769 (1971); and sum-frequency cross-correlation, as disclosed by K. M. Yoo et al. in Optics Letters, Vol. 16, p. 1019 (1991). Time-gated upconversion using pulses as short as 65 fsec has been used to measure biological specimens such as the corneal structure of rabbit eyes, as disclosed by Fujimoto et al. in xe2x80x9cFemtosecond optical ranging in biological systems,xe2x80x9d Optics Letters, Vol. 11, p. 150 (1986). In this method, the ranging was performed one point at a time, and required raster scanning of the beam over the specimen.
Subsequently, an optical coherence tomography (OCT) technique was disclosed by E. A. Swanson et al. in xe2x80x9cHigh-speed optical coherence domain reflectometry,xe2x80x9d Optics Letters, Vol. 17, p. 151 (1992), which employs only linear interferometry without any nonlinear optical interaction. Time-gated imaging by ultrashort pulses using second harmonic generation (SHG) was first disclosed by Diels et al. in xe2x80x9cImaging with femtosecond pulses,xe2x80x9d Appl. Opt., Vol. 31, p. 6869 (1992) and in xe2x80x9cUltrafast diagnostics,xe2x80x9d Revue Phys. Appl., Vol. 22, p. 1605 (1987). In this method, a gating pulse was used to time-gate and upconvert entire images of objects which were illuminated by an ultrashort pulse. However, this method does not provide any amplification of the image, and provides only a single contour or surface section.
Surface metrology measurement using ultrafast lasers in conjunction with sum-frequency mixing is disclosed in U.S. Pat. No. 5,585,913 to Hariharan, et. al., entitled xe2x80x9cUltrashort pulsewidth laser ranging system employing a time gate producing an autocorrelation and method therefor.xe2x80x9d In this method, a focused laser beam is scanned over the surface of the target in order to map out the surface topography. Again, in this method, there is no light amplification, and raster scanning is required to build up an image of a surface.
It is an object of the present invention to employ optical parametric amplification (OPA) in conjunction with conventional, Fourier or confocal imaging systems to achieve high gain and low noise amplification of signal light reflected from or transmitted through an object in order to produce an amplified image of the object.
It is a further object of the present invention to improve image resolution in confocal microscopy using optical parametric amplification.
It is another object of the present invention to use the time gating capability of optical parametric amplification to discriminate against scattered light.
A further object of the present invention is to use the time-gating capability of optical parametric amplification to provide optical sectioning of an object under test, similar to that obtained with optical coherence tomography (OCT).
A still further object of the present invention is to use the time gating capability of optical parametric amplification to produce a new method of fluorescence lifetime imaging.
Another object of the present invention is to use quasi-phase-matched nonlinear optic materials as the amplifying medium in an imaging system, thereby providing large angular acceptance and low pump thresholds.
Yet another object of the present invention is to lower the required excitation power of an illuminating beam in an imaging system, thereby allowing increased observation time, reducing photobleaching and enhancing the viability of cells being imaged.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
The present invention employs optical parametric amplification (OPA) in a nonlinear optical medium pumped by an ultrashort (less than 2 ns) pulse laser at frequency xcfx89p, to amplify and time-gate the scattered light from a target object illuminated by an ultrashort laser pulse at a signal frequency xcfx89s. In the process, another amplified signal is generated at the idler frequency, xcfx89i. This amplified light is recorded using a CCD camera or other imaging device. This technique can be used in conjunction with confocal imaging methods. By amplifying and time-gating the scattered light, optical sectioning of the object is achieved, enabling an image of an isometric contour of the object surface or interior to be produced. In the case of nondegenerate OPA, detection of the idler frequency instead of the signal frequency also achieves frequency conversion of the image. Standard gated image intensifiers (e.g., microchannel plates) have a time-gate window of approximately 100 ps which can resolve depth features with only approximately 1 cm resolution. By using ultrashort pulses (e.g., 100 fs) it is possible to resolve surface features with a resolution of approximately 10 microns. By using still shorter pulses, the longitudinal resolution improves further (e.g., down to 2 microns using 20 fs pulses).
Sum-frequency gating and Kerr gating yield comparable resolution when pumping with ultrashort pulses, but do not provide amplification, and, in fact, usually are most inefficient. Photon efficiencies typically do not exceed 10% with these systems. In contrast, by using ultrafast, time-gated, optical parametric image amplification (UTOPIA) it is possible to obtain both image amplification and time-gating simultaneously. The parametric amplification method of the present invention can be performed in collinear or noncollinear geometries, can be either degenerate or nondegenerate, and can employ type-I or type-II phase matching or quasi-phase matching.
Further, the technique of the present invention can be used in conjunction with either a confocal imaging system or a conventional or Fourier imaging system. If collinear, degenerate OPA is used, then the amplified contour image is superimposed on the unamplified image at the same frequency (since the idler frequency xcfx891 is the same as the signal frequency xcfx89s). This provides a convenient method of registration between the contour image and the visual image of the object. With degenerate OPA, the image amplification factor is sensitive to the relative optical phase between the pump and signal pulses. In some cases it may be desirable to obtain only the contour image with maximum discrimination against any background light. In these cases, it is advantageous to use nondegenerate UTOPIA which gives simultaneous image amplification, time-gating, and frequency conversion to the idler frequency. Illumination of the target with pulses at a wavelength near 1550 nm is particularly advantageous in many cases because this wavelength is considered to be eyesafe.
By illuminating the target with a single pulse, an isometric contour (or contours) corresponding to a particular depth level of the target surface (i.e., an optical section) is obtained. Then, by adjusting the optical path length (time delay) traversed by either the pump or signal pulses, a number of different contours can be obtained, whose spacings correspond to the adjustments in optical path difference. Thus, a multiple contour image can be built up from a number of single-contour images. If, instead, the target is illuminated by a sequence of N closely-spaced ultrashort pulses during the pump pulse period, then a multiple contour image with the contours corresponding to N different depth levels of the target surface is obtained with a single pump pulse. If the pump laser pulse is sufficiently powerful, then this multiple-contour image can be acquired using a single laser shot, making it possible to obtain topographic images of objects which are moving very rapidly, e.g., even at hypersonic velocities. While multiple-contour images have been obtained using interferometric methods, the contours so obtained are very closely spaced (e.g., at a fixed spacing of one wavelength of the light) which gives very high resolution, and which limits the total depth which can be probed with a CCD imaging system due to the finite number of pixels which comprise the CCD array. The UTOPIA system of the present invention can cover a large dynamic range in feature depth by adjusting the spacings between the optical pulses in the sequence. With resolution of 10 microns, it is still possible to map out a depth range of over 100 mm with no ambiguity.
The choice of the nonlinear optical medium for performing optical parametric amplification is an important aspect of the present invention. The advantages of using a noncritical phase matching geometry have been demonstrated in type I nonlinear crystals. Quasi-phase-matched crystals have significant advantages over type I and type II phase-matched crystals, as described by M. Yamada et al. in Appl. Phys. Lett., Vol. 62, p. 436 (1993). In particular, periodically-poled lithium niobate (PPLN) has a large nonlinear coefficient and can be tailored to the desired phase matching conditions, such as frequency and acceptance angle. PPLN enables noncritical phase matching, thus increasing the acceptance angle of the UTOPIA system. Thus, according to the invention, the nonlinear optical medium is preferrably a periodically poled ferroelectric optical material, including but not necessarily limited to lithium niobate, lithium tantalate, MgO:LiNbO3, KTP and crystals of the KTP isomorph family.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components.